1. Field of the Invention
The present invention relates to a disk system for recording and reproducing data by using a disk-like recording medium such as a magnetic disk and, more particularly, to a disk system for positioning a head on a data track in accordance with servo information formed on a portion of a sector on a data recording surface.
2. Description of the Related Art
In a conventional magnetic disk apparatus, "servo information" for "positioning" a magnetic head on a magnetic disk is previously formed so as to raise the density of tracks. In the magnetic disk apparatus, an accessing system for suitably positioning a magnetic head in accordance with the servo information is employed. "A sector servo system" is known as one type for positioning the magnetic head as described above. In the magnetic disk which employs the "sector servo system", servo information formed of a pattern repeated at a predetermined cycle is formed in advance on a portion of a sector (e.g., a servo sector) on the track. The servo information is read by the magnetic head, and a position error signal indicating a position error of a head with respect to a desired track is generated by sampling by using a sampling cycle to be determined in accordance with the number of sectors and the rotating speed of the disk. The position of the magnetic head is controlled so that the position error signal becomes minimum.
A small-sized hard disk apparatus or a floppy disk apparatus of magnetic disk apparatuses is frequently placed in a portable personal computer or the like. An external vibration of impact is easily applied to the apparatus. It is difficult to accurately follow a head to a mark track by holding a complete head positioning state against such a vibration or an impact. Therefore, the disk is heretofore constructed so as to obtain servo information in terms of data recording region (i.e., magnetic region) as many as possible thereby to widen the servo band, thereby improving "vibration-proof performance". The "positioning accuracy" is obtained by mounting the entire magnetic disk in a vibration removing mechanism.
In the above-described sector servo system, however, the position error signal of the magnetic head cannot be obtained between the sector and the sector. Thus, when the number of the servo sectors is suppressed so as to improve a format efficiency, a cycle in which the position error signal cannot be obtained is lengthened. Therefore, the follow-up performance of the magnetic head to a mark track (so-called track follow-up performance) against a disturbance such as an impact or a vibration or the responsivity of the magnetic head (i.e., disturbance suppressing characteristic) when the head is deviated from a mark track due to a disturbance, is deteriorated. As a result, there arises a problem that, since an accurate "positioning operation" cannot be maintained, positioning characteristic is lowered.
In a sample-data control system, a "phase lag" due to sample/hold generally occurs. In order to eliminate the influence of the phase lag to a control system, it is necessary to limit the servo band to approximately 1/7 or less of a sampling frequency. If the servo band is limited as described above, utility in a frequency band where "a disturbance" exists is lowered, and hence vibration-proof performance is deteriorated.
On the other hand, in order to position a head in a magnetic disk apparatus and particularly a hard disk apparatus, a "data surface servo system" represented by a "sector servo system" has been employed instead of a "servo surface servo system" heretofore frequently used. And the data surface servo system is also employed by a large capacity flexible magnetic disk apparatus in order to improve the track density. The data surface servo system does not used an exclusive use servo surface like the servo surface servo system, but writes (records) servo information on a data surface, and does not have a thermal offtrack which becomes a problem in the servo surface servo system but can obtain servo information (i.e, position information) directly from a data track. Therefore, the data surface servo system has an advantage that an accurate "head positioning" can be achieved.
In the conventional sector servo system, as shown in FIG. 4, a servo sector 33 is provided on a partial region of a sector on a data surface of a disk 1, i.e., between data sectors 32. A servo pattern 31 for applying head position information of radial direction of the disk is formed in advance on the servo sector 33. A head for recording and reproducing data intermittently read the servo pattern 31 at each servo sector 33. A position signal as shown in FIG. 5 is, for example, decoded from the information of the servo pattern 31 thus intermittently obtained, and the head is controlled to be positioned in accordance with the position signal dispersively obtained at each sector.
In a disk apparatus using such a sector servo system as a head positioning servo, or particularly in a large capacity flexible magnetic disk apparatus, it is necessary to obtain a sufficient servo band so as to follow up the head to "track radial runout" generated due to "eccentricity" generated upon elongation or contraction of a medium due to temperature and moisture changes or replacement of a disk (i.e., the track radial runout of a primary mode having a frequency equal to the rotating frequency of the disk or the track radial run-out of a secondary mode having twice the frequency) and to accurately position the head to a data track. Therefore, a wide servo band is realized to strengthen the head against a disturbance such as a vibration, an impact, etc., and to obtain faster "settling time".
The sector servo type disk apparatus has a "sampled-data control system". Therefore, in order to obtain a wide servo band, its sampling frequency may be set to a high value. As a method of obtaining a wide servo band by increasing the sampling frequency, there are generally the following methods:
(Method A): A method of accelerating the rotating speed of a disk
(Method B): A method of increasing the number of servo sectors (i.e., generally equal to the number of data sectors)
In the (method A) for accelerating the rotating speed of the disk, since the fluctuating frequency of the track is proportionally raised, the "follow-up performance of the head" against the fluctuation of the track is not improved. Therefore, in order to generally improve the performance of the disk system, it is most effectively to employ the (method B) to increase the number of servo sectors and to increase the quantity of the servo information. However, if the number of servo sectors is increased, better head positioning control is performed, but "format efficiency" (i.e., formatted capacity to unformatted capacity ratio) is reversely decreased. Thus, the storage capacity of the data to be actually recorded is reduced. Therefore, it is difficult to obtain sufficient storage capacity and to simultaneously attain the number of sectors to obtain a sufficient servo band only by the methods A and B.
In the conventional "sector servo system", as shown, for example, in FIG. 14, a servo pattern 31 repeated in a predetermined cycle radially of a disk is previously formed as "servo information" on a partial region of a sector on a disk, i.e., on a servo sector 33 provided intermittently between data sectors 32, the position signal of the head obtained by reading the servo pattern 31 is decoded to control to position a head 35. The servo pattern 31 has a position information portion 34, which has information bits A, B, C and D for obtaining two-phase position signals X and Y used to accurately follow up the head 35 to a data track, and information bits P, Q and R for enhancing the position detecting ability of the head 35 by using the position signals X and Y to detect the position even if the head 35 is moved faster.
FIG. 15 shows a position signal obtained by decoding the servo pattern 31. When the amplitude values of the signals obtained by reading the information bits A, B, C, D and P, Q, R by the head 35 as the head 35 passes over the servo pattern 31 are defined as SA, SB, SC, SC and SP, SQ, SR, respectively, X=SA-SB, Y=SC-SD are satisfied. The position signals X and Y are used as analog signals, while the SP, SQ and SR are binarized, and used as digital signals of "1" or "0". In the case of controlling to follow up a track, the position signals X and Y are used. The position where they become "0" is used as a data track center, a feedback control is performed so that X=0 or Y=0 is obtained, thereby following up the head to the data track. In the case of seeking the head to a target track, the position and the speed of the head are obtained from the signals X, Y, SP, SQ and SR, and the head is controlled at its speed in accordance with a desired speed previously set in response to the distance to a target track. A head position detecting method first determines the magnitude of the X and Y to determine the zone L0, L1, L2, L3 where the head exists. More specifically, it is determined in accordance with the criterion of the following formulae: EQU X.gtoreq.0, Y&gt;0.fwdarw.L0 EQU X&gt;0, Y.ltoreq.0.fwdarw.L1 EQU X.ltoreq.0, Y&lt;0.fwdarw.L2 EQU X&lt;0, Y.gtoreq.0.fwdarw.L3
Then, second position signal groups U=X+Y, V=X-Y are respectively obtained from the signals X and Y. Further, positions D0, D1, D2 and D3 in the zones L0, L1, L2 and L3 determined previously are obtained from the signals U and V as below, where .+-.a is the amplitude value of the signals X and Y shown in FIG. 15 EQU L0 : D0=V/(2a)+0.5 EQU L1 : D1=0.5-U/(2a) EQU L2 : D2=0.5-V/(2a) EQU L3 : D3=U/(2a)+0.5
In the above-described "head positioning method", the condition that the head is moved within four track zones including the zone where the head exists at present between samplings is set in advance, thereby detecting the head position without error.
For example, (a): in FIG. 15, the head is moved to the inside, and it is assumed that the head is not returned to the outside. At this time, if the head is in the zone Ln by sampling at an arbitrary servo sector, a range that the move can move up to the sampling in next servo sector is limited within four zones of the inside including the Ln (e.g., if Ln=L0, within the zones L0 to L3), and the position can be detected under the condition without error.
(b): It is assumed that the head is moved to the inside and the head might be returned at one track to the outside in the worst case. If the head is in the zone Ln by sampling at an arbitrary servo sector, a range that the head can move to the sampling in next servo sector is limited within three zones of the inside including one zone of the outside of the Ln and Ln (e.g., if Ln=L0, within the zones L13 to L0 to L2), and the position can be detected under the condition without error. Therefore, in the former case (a), the range that the head can move between continuous sampling points (i.e., time for detecting the two continuous servo sectors by the head, i.e., the time equal to the servo sector detecting cycle), i.e., an allowable maximum moving speed is limited to less than three track pitch per time equal to servo sector detecting cycle. In the latter case (b), the range becomes less than two track pitch.
When the signals of SP, SQ and SR having further longer repetition cycle are used together, sixteen track zones of L0 to L15 can be identified, and the position of the head within the sixteen tracks can be unitarily identified. In the case (a) of this case, the allowable maximum moving speed is limited to less than fifteen track pitch per time equal to the servo sector detecting cycle. In the case (b), the allowable maximum moving speed is limited to less than fourteen track pitch. Thus, the allowable value of the maximum speed of the head can be enhanced by adding information bits P, Q, and R to the position information portion 34 of the servo pattern 31. In order to obtain the position of the head, the "0" track is first detected, the present position of the head is determined from position information obtained by the previous sampling and the position information obtained by the present sampling by the restriction that the moving speed of the head is within the allowable maximum moving speed, the positions to be determined at each sampling from the "0" track position are accumulated and calculated to obtain the present position of the head.
As described above, the pattern repeated at a predetermined cycle is formed, and the position of the head can be unitarily identified within the cycle. Further, the present position of the head can be detected by the position obtained by the previous sampling and the servo information obtained by the present sampling by the restriction that the distance for moving the head between the two sampling points is within the cycle or less. This head position detecting method is known as so-called "a relative position detecting method".
On the other hand, as the other conventional head position detecting method, there is known an "absolute position detecting method" in which servo information such as a Gray code to be unitarily identified from servo information is formed when the servo information is detected irrespective of the information of the previous position where the present head is disposed at any track and at any position in the track.
However, according to the above-described relative position detecting method, the servo information may be less, but it has the restriction that the moving distance of the head between servo information sampling points must be within a range that the head can be unitarily identified, and hence the moving speed of the head is limited. In this case, if the repetition cycle of the servo pattern 31 is lengthened by adding position information bit having long cycle like P, Q and R of FIG. 14, the head position detecting ability is improved, and the head can be moved at faster speed, but since the maximum merit of this method i which the servo information amount may be less is lost, it has a limit. According to the above-described method, when the ranges where the head position can be unitarily identified from the servo pattern 31 are the same, the less the number of the sectors is, the more the allowable maximum moving speed is decelerated. Generally, since the number of sectors is determined by considering the other specifications such as data format, track follow-up performance, etc., the position and speed detecting ability, i.e., seeking performance depends upon the specifications, and it cannot be designed independent of them.
On the other hand, according to the absolute position detecting method, since it required the servo pattern having a number of bits such as Gray code, etc., the servo information amount is very increased as compared with the relative position detecting method. Therefore, it has a malfunction that data format efficiency is extremely wrong.
As described above, the sector servo system used to position the head in the disk apparatus such as the flexible magnetic disk apparatus (i.e., FD) lowers its data format efficiency if the number of servo sectors is increased so as to increase its servo band, and it is difficult to obtain sufficient storage capacity.
In the conventional sector servo type magnetic disk apparatus, for example, the following characteristics are provided:
(Characteristic 1): The position error signal of the magnetic head is not obtained between a sector and a sector.
(Characteristic 2): The number of servo sectors is limited in format efficiency.
(Characteristic 3): The open-loop gain of a servo system in a frequency band in which a disturbance exists is low.
The sector servo type magnetic disk apparatus is difficult by the above-described various characteristics to correctly follow u the magnetic head to a target
p track against a disturbance such as an external vibration or impact, and has a problem that its responsivity at the time of offtracking due to a disturbance is deteriorated.
Further, the "relative position detecting method" for detecting the present position of the head from the position obtained by the previous sampling and the servo information obtained by the present sampling of the head position detecting methods used in the conventional disk apparatuses may use less servo information, but has a restriction that the moving distance of the head between servo information sampling points must be within the range in which the position of the head can be unitarily identified. Thus, in order to accurately detect the position and moving speed of the head, the moving speed of the head is limited. Therefore, it has a problem that a high speed seeking is difficult to be realized. If the repetition cycle of the servo pattern is lengthened in this method, the head position detecting capacity is improved and the high speed seeking is performed, but the servo information amount is increased to lose its maximum merit.
In the "absolute position detecting method" for forming the servo information to be unitarily identified from the servo information irrespective of the previous position information of the present position of the head, it is necessary to use a servo pattern having a number of bits such as Gray code. Therefore, the servo information amount is very increased, and there is a problem that the data format efficiency is deteriorated.